EFFECTS OF CHROMOSOME UNDERREPLICATION ON CELL-DIVISION IN ESCHERICHIA-COLI

The key processes of the bacterial cell cycle are controlled and coord inated to match cellular mass growth. We have studied the coordination between replication and cell division by using a temperature-controll ed Escherichia coli intR1 strain. In this strain, the initiation time for chromosome replication can be displaced to Later (underreplication ) or earlier (overreplication) times in the cell cycle. We used underr eplication conditions to study the response of cell division to a dela yed initiation of replication. The bacteria were grown exponentially a t 39 degrees C (normal DNA/mass ratio) and shifted to 38 and 37 degree s C, In the last two cases, new, stable, lower DNA/mass ratios were ob tained, The rate of replication elongation was not affected under thes e conditions. At increasing degrees of underreplication, increasing pr oportions of the cells became elongated. Cell division took: place in the middle In cells of normal size, whereas the longer cells divided a t twice that size to produce one daughter cell of normal size and one three times as big, The elongated cells often produced one daughter ce ll lacking a chromosome; this was always the smallest daughter cells, and it was the size of a normal newborn cell. These results favor a mo del in which cell division takes place at only distinct cell sizes. Fu rthermore, the elongated cells had a lower probability of dividing tha n the cells of normal size, and they often contained more than two nuc leoids, This suggests that for cell division to occur, not only must r eplication and nucleoid partitioning be completed, but also the DNA/ma ss ratio must be above a certain threshold value, Our data support the ideas that cell division has its own control system and that there is a checkpoint at which cell division may be abolished if previous key cell cycle processes have not run to completion.